546 POPULAR SCIENCE MONTHLY. 



A CENTURY OF GEOLOGY. 



By Pkof. JOSEPH LE CONTE. 



\^Concluded. ] 



THE AGE OF THE. EARTH. 



UXTIL almost the beginning of the present century the gen- 

 eral belief in all Christian countries was that not only the 

 earth and man, but the whole cosmos, began to exist about six 

 thousand to seven thousand years ago; furthermore, that all was 

 made at once without natural process, and have remained sub- 

 stantially unchanged ever since. This is the old doctrine of the 

 supernatural origin and substantial permanency of the earth and 

 its features. Among intelligent and especially scientific men this 

 doctrine, even in the eighteenth century, began to be questioned, 

 although not publicly; for in 1751 Buffon was compelled by the 

 Sorbonne to retract certain views concerning the age of the earth, 

 published in his Natural History in 1749.* Remnants of the old 

 belief lingered even into the early part of the present century, and 

 may even yet be found hiding away in some of the remote corners 

 of civilized countries. But with the birth of geology, and espe- 

 cially through the work of Hutton in Scotland, Cuvier in France, 

 and William Smith in England, the much greater — the inconceiv- 

 ably great — antiquity of the earth and the origin of its present 

 forms, by gradual changes which are still going on, was generally 

 acknowledged. Indeed, as already said, this is the fundamental 

 idea of geology, without which it could not exist as a science. 



Geology has its own measures of time — in eras, periods, epochs, 

 ages, etc. — but it is natural and right that we should desire more 

 accurate estimates by familiar standards. How old, then, is the 

 earth, especially the inhabited earth, in years? Geologists have 

 attempted to answer this question by estimates based on the rates 

 of sedimentation and erosion, or else on the rate of changes of 

 organic forms by struggle for life and survival of the fittest. Physi- 

 cists have attempted to answer the same question by calculations 

 based on known laws of dissipation of energy in a cooling body, 

 such as the sun or the earth. The results of the two methods 

 differ widely. The estimates of the geologists are enormous, and 

 growing ever greater as the conditions of the problem are better 

 understood. Nothing less than several hundred million years will 

 serve his purpose. The estimates of the physicists are much more 

 moderate, and apparently growing less with each revision. The 



* Lyell's Principles of Geology, eighth edition, p. 41, 



